Increasingly higher demand is recently mounting for piston rings having improved scuffing resistance and wear resistance vis-à-vis cylinder liners, as higher-power, higher-fuel-efficiency internal engines are required. To satisfy such demand, piston rings are coated with chromium plating film. However, conventional chromium-plated piston rings fail to exhibit sufficient scuffing resistance and wear resistance, when used for engines subjected to high thermal load and severe wear, such as diesel engines and lead-containing gasoline engines.
Proposals were thus made to provide chromium plating films capable of retain oil for a long period of time by microcracks expanded not only on the surface but also inside. For instance, JP 10-53881 A (Reference 1) proposes a sliding member coated with a multilayer plating film having fine cavities on the surface and voids inside. The film was prepared by repeating a cycle of chromium plating and an inverse voltage treatment. If a sliding surface coated with multilayer chromium plating film is worn, voids distributed inside the film are exposed as cavities to a new surface. Accordingly, the sliding member can always maintain an appropriate amount of a lubricating oil on the sliding surface. However, the sliding member of Reference 1 is insufficient in wear resistance.
JP 62-56600 A (Reference 2) proposes a hard chromium plating layer containing solid particles in a crack network extending through the entire thickness. JP 10-130891 A (Reference 3) proposes a composite chromium plating film containing Si3N4 particles having an average diameter of 0.8 to 3 μm in a network of cracks, with a composite ratio (percentage of hard particles in the film) of 3 to 15% by volume. JP 10-130892 A (Reference 4) proposes a composite chromium plating film containing sphere particles having an average diameter of 0.7 to 10 μm in a network of cracks, with a composite ratio of 3 to 15% by volume. Because the plating films of References 2-4 contain solid particles in cracks, the cracks are prevented from being closed by sliding stress, resulting in improved wear resistance.
However, because the chromium plating films of References 2-4 are formed by using a conventional sergent bath or silicofluoride bath, they have small distribution densities of microcracks, which are expressed by the number of microcracks crossing a line of unit length. Because the plating film with a small distribution density of microcracks fails to sufficiently retain a lubricating oil on a sliding surface, it does not have good, stable sliding ability. The distribution density of microcracks can be increased to about 2,000/cm by the conventional plating method using the sergent or silicofluoride bath, as long as suitable plating conditions are selected. However, a plating film thus obtained has microcracks with inhomogeneous distribution density, and part of the plating film has low strength and wear resistance, resulting in difficulty to put such plating film in practice use.
Reference 1 JP 10-53881 A,
Reference 2 JP 62-56600 A,
Reference 3 JP 10-130891 A, and
Reference 4 JP 10-130892 A.